They can help and won't cause any issues, but it depends on how bad your system is. No worries running them but if the OS is running extremely slow then you are better off doing a clean install.
Also depends on the OS, Windows 7 and 8 will perform Defrag in the background, using CCleaner is also a good option

I think it's worth having Defraggler as visual aid to check that Win Defrag, which doesn't have a graphic, is working as desired and also it's a good on-demand defragger for when Windows been doing service packs, updates etc. ...this minimises the resources* used when it's on in the background if it's going to be on a long time like when it's quite heavily fragmented after a big update etc. Also, the time you are using the computer may not synchronise with the defrag schedule sometimes so a quick manual check and defrag with Defraggler can get it done quickly. The great majority of the time, just let Win Defrag do it's stuff and forget about it.